Low smoke combustion system

ABSTRACT

A COMBUSTION SYSTEM FOR A GAS TURBINE WHICH PRODUCES A SMOKELESS EXHAUST STACK IS CONSTRUCTED SUCH THAT THE COMBUSTION REACTION ZONE WILL OPERATE WITH A LEAN FUEL-AIR MIXTURE. THE LEAN COMBUSTION ZONE IS STABILIZED BY A VORTEX GENERATED AROUND THE CENTERLINE OF THE LINER, CREATING GAS FLOW PATTERNS THAT CAUSE STRONG BACK FLOW IN THE COMBUSTION REACTION ZONE. THIS VORTICAL FLOW ALSO SWEEPS OUT THE FUEL-RICH POCKETS, THUS FACILITATING COMPLETE COMBUSTION. A THERMAL SOAKING REGION WITH NO AIR HOLES IN THE LINER IS PROVIDED DOWNSTREAM OF THE LAST ROW OF COMBUSTION AIR HOLES HAVING AN AXIAL LENGTH OF AT LEAST 1.25D WHEREIN THAT SOOT WHICH MAY BE FORMED IN THE COMBUSTION PROCESS IS CONSUMED BY A VERY FAST CHEMICAL REACTION. THE TEMPERATURE THROUGHOUT THE SOAKING REGION IS MAINTAINED SUFFICIENTLY HIGH TO INDUCE SUCH A REACTION. AXIALLY DOWNSTREAM OF THIS REGION, TEMPERING AIR IS ADDED TO THE COMBUSTION PRODUCTS SUCH THAT THEY ARE COOLED SUFFICIENTLY IN ORDER TO ENTER THE FIRST STAGE NOZZLE AT THE DESIRED TEMPERATURE.

Sept. 28, 1971 w. E HILL ETAL 3,608,309

Low SMOKE COMBUSTION SYSTEM Filed May 21, 1970 3 Sheets-Sheet 1 TO FIRSTSTAGE NOZZLE\ FIGJ m vcomausnom *Pnoouc'rs FLOW 1.250

INVENTORS: WILLIAM E. HILL, MILTON B.HILT,

EDWARD I. HOPKINS, ROBERT H. JOHNSON,

HEIR ATT RNEY Sept. 28, 1971 w. E. HILL ETAL LOW SMOKE COMBUSTION SYSTEM3 Sheets-Sheet 2 Filed May 21, 1970 o. M C. T P m P v A m R E m L T P AIS m D A R R E m L I I I I l E R m mmnmmmmm az tood B F NO VISIBLE8Mi)KE OF TOTAL COMBUSTION AIR FLOW AREA mmmzDz wxOEw 024mm 20 LII smmmRHHO m5 W mm U H MD E IN THE COMBUSTION REACTION ZONE N Y. E N N H R 0 OH T m H E w T R Sept. 28, 1971 3,608,309

W. E- HILL EI'AL LOW SMOKE COMBUSTION SYSTEM Filed May 21. 1970 3Sheets-Sheet 5 \\\\\\\\\\\\\\\\\\\\l l\\\\\\\\\\\' X 1" CI mvsmons:WILLIAM E. HILL, MILTON a. mu, EDWARD P. HOPKINS, no RT H. aormsou I EIRATTO 3,608,309 LOW SMOKE COMBUSTION SYSTEM William E. Hill, Scotia, andMilton B. Hilt, Edward P. Hopkins, and Robert H. Johnson, Schenectady,N.Y., assignors to General Electric Company Filed May 21, 1970, Ser. No.39,332 Int. Cl. F02c 3/24; F23r 1/10 US. Cl. 60--39.65 4 Claims ABSTRACTOF THE DISCLOSURE A combustion system for a gas turbine which produces asmokeless exhaust stack is constructed such that the combustion reactionzone will operate with a lean fuel-air mixture. The lean combustion zoneis stabilized by a vortex generated around the centerline of the liner,creating gas flow patterns that cause strong back flow in the combustionreaction zone. This vortical fiowalso sweeps out the fuel-rich pockets,thus facilitating complete combustion. A thermal soaking region with noair holes in the liner is provided downstream of the last row ofcombustion air holes having an axial length of at least 1.25D whereinthat soot which may be formed in the combustion process is consumed by avery fast chemical reaction. The temperature throughout the soakingregion is maintained sufiiciently high to induce such a reaction.Axially downstream of this region, tempering air is added to thecombustion products such that they are cooled sufiiciently in order toenter the first stage nozzle at the desired temperature.

BACKGROUND OF THE INVENTION The present invention relates to gas turbinecombustion systems of the can type, and more particularly, to acombustion system which is smokeless throughout its operating range.

A negative result of todays mechanized society is the ever-increasingproblem of air pollution. Many of todays fuel-burning machines exhaustpollutants into the air causing a variety of harmful effects to naturesbalance. The industrial gas turbine is no exception and efforts havebeen made to reduce the smoke production in such gas turbines in orderto limit the amount of this pollutant exhausted into the atmosphere.

The operation of a gas turbine combustor and the combustion process iscomplicated; however, it is known that the amount of smoke emitted fromgas turbines depends upon two criteria. One criteria for the amount ofsmoke or soot produced depends upon the amount of incomplete combustionand fuel cracking in the rich combustion reaction zone or region of thecombustor. The second criteria,

which heretofore has not been considered, is the amount of this sootthat is consumed, that is, eliminated, in high temperature chemicalreactions subsequent to the combustion in the reaction zone. These twocriteria present themselves such that an ideal combustion system wouldkeep the soot production to a minimum and the subsequent sootconsumption at a maximum.

The gas turbine exhaust is relatively free of air pollutants because thecombustion process is carried out with air greatly in excess of astoichiometric mixture and is comparatively complete. In an ideal hotoxidizing atmosphere, using a hydrocarbon fuel combusted with air, thefollowing generalized formula presents itself for perfect and completecombustion: C H +O co,+H,o, yielding a smokeless exhaust stack. In theother extreme of a hot inert atmosphere, where there is incompletemixing of the hydrocarbon with air, the following chemical reaction(thermal cracking) takes place: C H C+C H yielding carbon atoms andsubsequently polyacetylenelike compounds which coagulate to form sootparticles on w'nited States Patent the order of one micron in diameter.Most combustors operate in a range between the two above cited limitingreactions. The soot particles formed in a less than perfect combustorare generated in fuel-rich regions that may exist in the combustionreaction zone and they subsequently grow in regions that are notconducive to further oxida tion in the post reaction zone. To oxidizethe soot particles in the post reaction zone, the following reactioncould occur: C+20 CO which is too slow to yield the desired result inthe short residence time in a gas turbine combustor. If the temperatureis high enough in the post reaction zone of the gase turbine combustionchamber, water vapor will dissociate as: 2H O H +2OH, thus forming apair of negative hydroxyl radicals. These highly reactive hydroxylradicals then combine with the soot as in the following reaction: C+2OHCO +H which is a sufliciently fast reaction for the time involved. It isthis last reaction that consumes the soot in the present combustionsystem.

In the first instance, the rich fuel pockets should be eliminated inorder to keep the soot production to a minimum. It is known to the priorart that by leaning. out the combustion reaction zone, this may, to acertain extent, be accomplished. When leaning out the combustionreaction zone, the flame stability is decreased as the minimum fuel/airratio is approached, thus indicating the necessity for providing meansto maintain the flame stability.

It has been suggested in the prior art that by imparting a swirlingmotion to a portion of the incoming combustion air, the stability of theflame will be increased. We allow additional air to be added so as tofurther lean out the combustion reaction zone. Here, this vortex flow isalso used to create a well mixed reactor by sweeping out the fuel richpockets.

The problems of trying to reduce smoke associated with the prior artwere essentially twofold. Firstly, additions were made to thegas turbinecombustion system, such as additives in the fuel (for example,manganese) and cumbersome and complex air scrubbers which acted directlyon the gas turbine exhaust. These additions to the combustion systemusually represented both increased cost and lower performancecharacteristics. In the second instance, it was oftentimes felt that theadded expense and complexity of such a combustion system did not warranttheir use on a particular gas turbine, thereby simply allowing the smoketo exhaust into the arr.

The smoke density in the gas turbine exhaust may be measured as the VonBrand reflective smoke number which is determined by drawing turbineexhaust gas at a specified rate of flow through a strip of filter papermoving at a fixed rate. The smoke trace produced on the filter paper isevaluated by measuring light reflectance using a photometer. The VonBrand smoke numbers range from 0 to 100 with 100 being the reflectanceof a clean tape. A Von Brand smoke number of and above is generallyconsidered a clear exhaust. In the prior art, by using additives and airscrubbers, it was possible to increase the Von Brand smoke number to thelow 90s at high loads.

Also used in the prior art to reduce the smoke of a gas turbine were airswirlers or vortex nozzles. The air swirler was used alone to impart aswirling motion to the incoming combustion air such that many of thefuelrich pockets would be eliminated, thus adding to completecombustion. The optimum design for one air swirler relating to smokeelimination is the subject of a separate patent application, Ser. No.7,947, filed on Feb. 2, 1970 in the name of Edward P. Hopkins andassigned to the assignee of the present invention. It is the particulardesign of the air swirler, which is described in the aforementionedpatent application, together with the further subject matter to bedescribed herein, which effectively eliminates smoke and increases theVon Brand smoke number well into the 90s. By leaning out the combustionreaction zone with a certain percentage of combustion air, imparting anoptimum swirl, and then providing a post reaction zone where there areno air holes such that a thermal soaking region is formed, the maximumreduction of smoke in the exhaust stack can be realized.

Accordingly, the primary object of the present invention is to reducethe amount of smoke exhausted to the atmosphere.

Another object is to decrease the smoke production without a sacrificein performance over the entire load range of a power generating gasturbine.

Still a further object as to improve combustion stability so as toextend the operating range of the system.

SUMMARY OF THE [[NVENTION Briefly stated, the present invention ispracticed in one form by providing a gas turbine combustion chamber witha lean combustion reaction zone which is stabilized by a vortex formedthrough an air swirler having 3 to 10% of the total combustor open areaformed by blades of critical thickness and critical angle. Combustionair holes are provided in the liner which are then followed by anaxially extending thermal soaking region of at least 1.25 times thediameter of the combustor liner in which only metal cooling air isprovided. A set of large holes is then provided for the entry into thecombustion liner of the final cooling or tempering air. It is the airswirler which provides the required flow and mixing capabilitiestogether with a strong feedback in the combustion reaction zone thatincreases the combustion stability so that practically no soot isproduced. The region with no air holes along the combustion linerprovides the high temperature region (thermal soaking) where the sootparticles that may have been produced in the combustion reaction zoneare consumed in high temperature fast chemical reactions. A balance ofthe parameters making up the system is necessary in order toaccomplishthe objects of the invention.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a view in section of atypical gas turbine combustion chamber showing the present invention.

FIG. 2 is a. View of the face of the air swirler which is mounted aroundthe fuel nozzle in the head end.

FIG. 3 is a view taken along lines III-III of FIG. 2 and shows asectional view of the air swirler.

FIG. 4 is a partial view of the swirler blades and slots taken alonglines :lV-IV of FIG. 3 and indicates some of the critical dimensions ofthe air swirler.

FIG. 5 is a graph showing the variation in velocity and pressure as thedimension outwardly from the liner center line increases.

FIG. 6 is a graph showing a curve which indicates the change in VonBran-d smoke number as the total flow area in the combustion reactionzone is varied.

FIG. 7 is a detailed view of the air swirler and fuel nozzle in the headend showing the general flow pattern.

DESCRJPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. *1, atypical combustion chamher for use in a gas turbine is generallyindicated at 1. Combustion chamber 1 is of the type where the compressedair from the compressor (not shown) is directed in a reverse flow. Thereverse flow of such a combustion chamber is well known in the art andprovides the advantage of heating the compressed air before its use inthe combustion processes.

Combustion chamber 1 is comprised of a generally cylindrical outercasing 2 to which is attached the casing 3. Casing 3 in turn connectswith the turbine section (not shown). The outer casing end cover 4closes olf the end of outer casing 2 opposite the casing 3 such that thevolume within outer casing 2 is sealed from the atmosphere. Extending ina generally axial direction and generally coaxial with outer casing 2,is the combustor liner 5 having a diameter indicated as D. As is wellknown in the art, it is within the liner 5 at the head end or combustionreaction zone 6 where the combustion process takes place in an operatingcombustor for gas turbines. As the hot combustion products proceedthrough the cylindrical liner 5, they are tempered with diluting air.They then reach the transition liner 7 which directs the temperedcombustion products to the first stage nozzle (not shown). An annularair space 8 surrounds the liners 5 and 7 in order to accommodate theflow of the com pressed air.

Generally closing off the end of liner 5 toward the outer casing endcover 4 is the liner end cap 9 which accommodates the fuel nozzlegenerally indicated as 10'. The liner end cap 9 is generally in theshape of a truncated cone, the top of which is for the accommodationtherein of the fuel nozzle assembly 10. The air swirler assemblygenerally indicated as 11 in FIGS. 2 and 7 is attached to the cap 9 inthe preferred embodiment but may also be attached to the fuel nozzle.The fuel nozzle assembly 10 may be any convenient type known to the artwhich can be accommodated in the head end of the liner 5 andparticularly in the end cap 9. Fuel nozzle 10 is of the variety which iscapable of atomizing hydrocarbon fuels, that is, those fuels which tendto create smoke in the gas turbine exhaust. The fuel nozzle 10 may be ofthe air atomizing'type or pressure atomizing type, both operatingequally well in this combustion system.

It is known in the art that the liners of combustion chambers areprovided with spaced holes for the entry thereinto of the air whichsupports the combustion and also cools and dilutes the products ofcombustion. We have found that by positioning these holes in a certainmanner along the length of liner 5, the object of reducing the smoke inthe gas turbine exhaust may be realized. It has also been found that thecombustion process takes place in combustion reaction zone 6 which isdefined approximately by an axial length which is equal to the diameterdimension (D). That is, within the axial length corresponding to theliner diameter (D), the fuel-combustion air mixture is burned, to eitherthe point of complete combustion or if incomplete combustion, formingthe soot particles. In the combustion reaction zone 6, there arepositioned two rows of combustion air holes. Although two rows areshown, this is not to be taken as an upper or lower limit.

Referring to FIG. 6 in addition to FIG. 1, it will be seen that thecurve in FIG. 6 indicates that as the percentage of combustion air flowarea in the combustion reaction zone increases, so too does the VonBrand smoke number, and as previously mentioned, an increase in thissmoke number indicates a decrease in the amount of smoke in the gasturbine exhaust. It is the first two rows of air holes in the liner 5,together with the open area of the air swirler 11, that constitutes thecombustion air flow area in the combustion reaction zone 6. As will bedescribed later, when referring to the details of air swirler 11, theopen area of the air swirler 11 must be maintained at between 3-10% ofthe total air flow area, which is defined here as the combustion airflow area, plus the area of the downstream tempering holes plus theliner open area comprising the cooling louvers (not shown). As such, therows of combustion air holes in the liner 5 located in the combustionreaction zone 6 should provide approximately 25-50% of the total airflow area into the liner 5. As indicated on FIG. 1, there are two rowsof combustion air holes. A first row 12 is comprised of 8 holescircumferenti-ally spaced about the liner 5, and have a hole diameter toliner diameter ratio of about .075. A second row 14 is again comprisedof 8 holes circumferentially spaced about liner and have theaforementioned ratio on the order of .12. Rows 12 and 14 are axiallyWithin the 1D dimension, that is, they are within combustion reactionzone 6.

Following the row of holes 14 downstream (in relation to the flow ofcombustion products) in an axial direction, is the thermal soakingregion of the liner 5. This is indicated as 15 on FIG. 1. The thermalsoaking region 15 is closed in that there are no large circumferentiallyspaced holes along this axial length of liner; however, louvers or slitsfor metal cooling air are positioned throughout the length of liner 5,but are not shown for clarity. The louvers are utilized for cooling theliner 5 and the air which enters the louvers does not contribute to thecombustion process to an important degree. The soaking region 15 must beat least 1.25D in axial length. It is in the region 15 where the thermalsoaking of the combustion products takes place, that is, it is herewhere, if the temperature is high enough, the following chemicalreactions will occur: 2H O H +2OH- and thereby consuming the sootparticles of carbon and carbonaceous material which are produced by theincomplete combustion or thermal cracking in the reaction zone 6. Theaxial length of at least 1.25D is provided so that a sufficient time isallowed for these reactions to take place.

Positioned at the end of the thermal soaking region 15 are a pluralityof circumferentially spaced tempering air holes 16. In FIG. 1, holes 16are comprised of four circumferentially spaced holes having a holediameter to liner diameter ratio of about .20. These dimensions aregiven by way of example only and should not be taken as an upper orlower limiting value. The actual size and number of tempering air holes16 will depend upon the amount of tempering air to be added to thecombustion products as they leave the soaking region 15. The temperingregion of the liner 5 is indicated on FIG. 1 as 18 and extends generallyfrom the tempering air holes 16 to the first stage nozzle. The purposeof the tempering air holes 16 is to allow a portion of the compressedair which is relatively cool as compared to the hot combustion productsto temper the combustion products before the overall air-combustionproduct mixture enters the first stage nozzle. Tempering holes 16 arelarge enough to allow suflicient penetration of the cooler tempering airinto the combustion products so that the desired first stage turbineinlet temperature is achieved.

Turning now to a detailed description of the air swirler 11, one exampleof which may be found in the aforementioned copending application of E.P. Hopkins (Ser. No. 7,947) reference will be made to FIGS. 1 through 4.It is air through swirler '11 which provides the necessary stabilizingeffect to allow a lean combustion zone to be formed as well as theswirling air to sweep out the fuelrich pockets in the combustionreaction zone 6.

Air swirler 11 is comprised of an annular body portion 17 which definesa centrally positioned hole 21 for the accommodaton therein of the fuelnozzle asernbly 10. The face side 19 of the body portion 17 is that sidewhich looks directly into the cylindrical liner 5 and which faces thecombustion process.

Spaced about the circumference of annular body portion 17 are aplurality of blade members 22. Blade members 22 may be formed in thebody portion 17 by any suitable means such as by machining or casting.Surrounding the blade members 22 is an annular shroud band which servesto provide an attachment member when the swirler 11 is secured to theend cap 9.

There are certain critical dimensions which must be maintained whenconstructing the blades 22. The first critical dimension is that of theslots 23 which are formed between adjacent blade members 22. The totalflow area on a plane perpendicular to the axis of the slot should befrom between 3 to 10% of the total air flow area into the cylindricalliner 5. This slot area is established by having certain dimensionalratios fall within certain limits. The first such dimensional ratio isdefined as the length of a slot 23 (indicated as the letter A on FIG. 4)to the width of the slot (indicated as B on FIG. 4). The ratio, that is,A/ B, is such that it falls within a range of from 1.15 to 1.85; or thelength is less than twice the slot width.

The blade thickness is critical at the trailing edge surface 24, and itshould be noted that this thickness is substantial as compared to priorart air swirler blades and is on the order of from .4 to .8 times theswirler slot width B. The trailing edge surface 24 is provided with acomparatively large dimension in order to provide an inward flow pathfor the air in the wakes which are generated by that portion of the hairflowing through the swirler slots. This inward flow may be seen byreference to the flow arrows in FIG. 7.

Another of the critical dimensional ratios provided on the air swirler11 in order to accomplish the objects. of the invention is that of thedepth of a slot 23 (indicated as C on FIG. 2) to the blade thickness(indicated as T on FIG. 2). Thisratio C/T, must be maintained within therange of 1.5-3.5 as it is this ratio which helps to determine the radialinward flow of hot gases in the stagnant wakes behind the relativelythick trailing edges. It will, of course, be realized that the angle 00of an individual blade member 22 will determine the strength of thevortex. This angle is indicated on FIG. 4 and must fall within a rangeof from 25 to 35 with 30 being the preferred angle for optimum smokelessoperation.

Having described the structural elements which must be combined togetherin order to accomplish the objects of the present invention, that is,decreasing smoke so that the Von Brand smoke number will be well aboveover the entire gas turbine load range, the combustion system 1 will nowbe described in terms of its operation.

OPERATION OF THE INVENTION During operation, compressed air from thecompressor will fill the annular air space around the liners and end capsuch that as the air enters the various openings in the liner, apressure drop is apparent and necessary for operation. Starting with thecompressed air flow through the air swirler slots in the combustionreaction zone, the air flow and attendant flow of combustion productswill be described at various points along the axial length of the liner.

As approximately 3 to 10% of the air passes through the air swirler, itis imparted with a swirling motion such that a vortex is formed in aportion of the liner volume. This will be apparent in referring to thevarious air flows shown in FIG. 7. The characteristics of the vortex maybe seen when referring to FIG. 5 where the variation in velocity andpressure is plotted against the increasing radial distance from theliner centerline out to the liner. In the core area, that is, thecentral part of the swirling air, it is seen from the curves that thepressure is at its lowest while the velocity is at its highest. Thisindicates that, in a zone down stream from the air swirler, the pressureat a larger radial dimension is increased over the pressure in the corewhich is at a pressure level below that in the liner volume in general.The result of this is that the air which enters the liner through thecombustion air holes will penetrate the swirling core air and tend toflow toward the face of the air swirler and fuel nozzle. It will beappreciated that such a feedback effect adds to the overall mixing ofthe combustion air with the fuel. The incoming air (from the holes)which penetrates the swirling core, of course, mixes with the atomizedfuel, and then begins to react with the fuel. The effect of the swirlingair, together with the feedback effect, stabilizes the flame over thefull operating range and thus allows the additional air, which is addedto the combustion reaction zone to form a stabilized lean head end. Theaddition of more air will tend to prevent the formation of the unwantedsoot particles by allowing more complete combustion in the reactionzone. Furthermore, the vortical mixing will sweep out any fuel richpockets that may have formed.

The various pressure drops across the liner and to the centerline of theliner are indicated on 'FIG. 5. The top line indicated p is the pressurein the annular air space before the pressure drop across the liner whichis indicated as Ap The decrease in pressure inwardly from the liner isthen indicated by the sloping curve p and the full pressure drops fromthe annular air space to the centerline of the liner is indicated as APNow, as the combustion air has been mixed with the fuel and a stabilizedflame formed, with the attendant combustion products being formed, thethermal soaking region of the liner is operative. Any soot which mayhave been formed in the combustion reaction zone due to inadequatemixing or fuel-rich pockets which were not swept away by the swirlingmotion of the air is allowed to react with the hydroxyl radicals in highconcentration at elevated temperatures.

At an axial point at least 1.25D from the last row of combustion airholes, tempering air is added to the combustion products in order tocool them to a point where they may enter the first stage nozzle at thetemperature required in the gas turbine cycle and at a value that willnot harm the ensuing hot gas path parts.

It "will thus be appreciated that a gas turbine combustion system hasbeen described which produces a minimal amount of smoke pollutant toexhaust into the atmosphere. This is accomplished through the use of anair swirler which provides the maximum mixing capabilities of thecombustion air with the fuel by stabilizing the flame so that a leanhead end can be formed, and by providing a thermal soaking region alongthe liner such that any soot which is formed in the combustion processis consumed by a fast chemical reaction.

What is claimed is:

1. In a gas turbine combustion chamber, the combination of:

(a) an outer casing closed by an end cover at one end and leading to thefirst stage nozzle at the other end,

(b) a liner surrounded by said casing and having a generally circularcross section with said casing and liner together defining an annularair space therebetween,

(c) a cap closing off one end of said liner and defining a hole therein,

(d) an air swirler disposed in the hole in the cap and comprising:

(1) an annular body portion having a face normal to the axis thereof anddefining a central hole for the accommodation therein of a fuel nozzle,and

(2) a plurality of angled blade members disposed about the circumferenceof said body, each having an average thickness greater than one-fourththe depth, arranged and sized so that the open area between blades is310% of the total air flow area in said liner,

(e) a plurality of combustion air holes defined by said liner andpositioned within an axial length corresponding to the liner diameter,said holes arranged and sized such that they define 25-50% of the totalair flow area in said liner,

(f) a thermal soaking region extending from said combustion air holesaxially at least 1.25 times said liner diameter dimension and void ofany combustion air holes, and

(g) a plurality of tempering air holes positioned downstream of saidthermal soaking region, with respect to the flow of combustion products,arranged and sized such that they define 35-55% of the total air flowarea in said liner.

2. A gas turbine combustion chamber according to claim 1 wherein saidangled blade members on the air swirler are at an angle of from between25 to 35 measured from a plane parallel to the swirler axis.

3. A gas turbine combustion chamber according to claim 1 wherein saidair swirler is attached to the cap.

4. A gas turbine combustion chamber according to claim 1 wherein saidair swirler is attached to the fuel nozzle.

References Cited UNITED STATES PATENTS 2,398,654 4/1946 Lubbock 39.652,638,745 5/1953 Nathan 6039.65 3,099,910 8/1963 Schirmer 603 9.653,490,230 1/1970 Pillsbury 60-39.65 3,498,055 3/1970 Faitani 6039.652,586,751 2/1952 Watson 6039.65 3,447,317 6/1969 Dakin 60-3965 DOUGLASHART, Primary Examiner U.S. Cl. X.R. 431-352

